This invention relates to a device and method for applying an electric field, and methods of manufacture thereof. The invention is particularly suited for smoothing the shape of electric fields applied to microfluidic devices or “lab-on-chip” type devices (“LOCs”).
Microfluidic devices such as LOCs have widespread applications. An increasing number of such applications relate to the sequencing and sorting of objects such as biomolecules, and the sorting of cells. Conventionally, the sequencing and sorting of biomolecules, and the sorting of cells, is carried out using electrophoresis. Electrophoresis techniques are well known and are often used to separate objects (sometimes referred to as ‘analytes’) according to their electrical and hydrodynamic properties. Other separation techniques include the use of centrifugal spectrometers as described in EP1455949.
In conventional electrophoresis, a constant and uniform electric field is applied to move objects through a fluid or another sieving matrix. As they move through this material, the objects experience forces which depend on their shape and size (e.g. hydrodynamic forces) and/or on their affinity for the material (e.g. chemical attraction/repulsion forces), and an electric force due to the applied field, which depends on their charge. As a result of the different forces experienced by each object type, the objects move with different terminal velocities depending on their individual characteristics and thus they separate into “bands”.
In recent years, the concept of field shifting analysis for separation of objects has been proposed by one of the present inventors, wherein, rather than being constant, the applied electric field has a time dependent field gradient. Examples of electrophoresis devices which use this concept are described in WO 2006/070176, the entire content of which is hereby incorporated by reference. In comparison to conventional techniques, field shifting analysis offers enormous potential in terms of analytical and processing capabilities, offering several orders of magnitude faster and more sensitive separations.
Field shifting devices usually employ a network of electrodes to apply a suitable time dependent electric field gradient for the separation and manipulation of analytes and other materials in a microfluidic environment. For example, the microfluidic environment may involve a planar separation channel in or on a glass device, with cross sectional dimensions of the order of 0.1 to several hundred micrometers and a length of at least 500 μm.
Further examples of different electrophoresis devices can be found in U.S. Pat. No. 6,277,258 and US-A-2002/0070113.
In known microfluidic devices, including field shifting devices, the electric field is usually applied directly to the channel, via internal electrodes. This arrangement facilitates the generation of high electric fields by generating an electric current in a conducting separation buffer inside the channel. However, this configuration often leads to significant distortions to the electric field shape at the locus of each electrode along the channel. Accordingly, the field in the channel does not follow a smooth transition from high to low, as is desirable when implementing the field shifting technique for example, but instead consists of a series of steps. The separating molecules pass at very close proximity (contact) to the electrodes, “feeling” the field distortions and thereby degrading the resolution of separation. Similar problems are also encountered in other applications where it is desired to apply a shaped (i.e. non-uniform) electric field to a channel.
To address this problem, it has been proposed to increase the number of electrodes periodically positioned along the channel. However, in practice, such configuration does not completely reduce the electric field distortions for two reasons. The first reason is that it is impossible to position an infinite number of independently addressable electrodes along the channel. The second reason is that, since the electrodes have finite sizes, the voltage in the space immediately adjacent to the electrodes is constant (at a value approximately equal to the voltage of the electrode). Accordingly, the resulting electric field is zero. This can cause significant distortion in the overall electric field.
Another problem with conventional separation techniques, including known field shifting techniques, is that some analytes can be lost as rather than travelling past the electrodes as intended, the objects may travel towards the electrodes directly contacting the channel and effectively be removed from the separation process. Furthermore, gasses produced by electrolysis due to contact between the electrodes and the (typically aqueous) fluid in the channel enter the channel where they disrupt the electric field and the analysis.
Accordingly, there is a need for a technique which addresses the above issues.